The present invention relates generally to thermally conductive polymer compositions having very low thermal expansion properties. More specifically, the present invention relates to a thermally conductive polymer composition suitable for use as a substrate in devices that require high dimensional stability, wherein the polymer is characterized as having a coefficient of thermal expansion (CTE) that closely matches the CTE of ceramic materials. The principal benefit to the present invention as compared to the prior art is that the polymer composition can be molded over the heat-generating article so that the composition encapsulates the article. In other instances, the polymer composition, itself, can be molded into a separate component such as a heat sink or electronics substrate. Then, the molded component can be attached, fastened, bonded, or otherwise adjoined to the heat-generating article.
There are numerous devices such as electrical motors, ink-jet/laser printer heads, semiconductors, microprocessors, circuit boards, transistors, resistors, and the like that generate a substantial amount of heat during their operation. The heat must be removed in order for the device to function properly. It is known to mold a thermally conductive plastic material over such heat generating devices in some instances. Generally, the difficulty in the prior art arises from the fact that such over-molded thermally conductive polymer materials have a CTE value that is substantially different than the CTE of the encapsulated device. These different CTE values can lead to undesirable mechanical forces or a transfer of stress resulting in the differential expansion between the substrate and the thermally conductive polymer. For example, during operation of the underlying electronic device heat is generated, as a result of this heating process, the over molded plastic material will expand at a different rate than the rate at which the encapsulated device expands. The molded plastic material and encapsulated device will also contract at different rates once the heating process ends and the assembly begins to cool. These different expansion and contraction rates lead to the introduction of a great deal of localized mechanical stresses being introduced into the assembly.
There have been numerous laminated compositions that have been manufactured in an attempt to constrain the CTE of the resultant composition. In these compositions, laminate structures contain networks of fiber reinforcing that serve to constrain movement as a function of increasing temperature. The difficult found in this process is that laminate construction is basically a two dimensional process that greatly limits the potential geometry of the finished article.
In other cases, molded polymers having constrained CTE properties have been produced using fibrous reinforcement. This method however is mostly impractical because molding operations require a flow pattern that results in orienting the fibers along the flow patterns within the mold thereby creating a large CTE anisotropy wherein the finished part has a great differential in CTE properties within itself. Specifically, the CTE properties are reduced along the flow directions wherein the reinforcing fibers are aligned, but the CTE remains relatively large across the flow directions within the part.
Some prior art attempts have been directed toward creating a molded polymer composition having reduced CTE values that are not subject to the anisotropy identified above. In order to overcome some of the stresses created by this CTE differential between the substrate material and the over molded thermally conductive polymer material, these attempts have eliminated the use of fiber reinforcing in favor of creating a polymer composition that has a native CTE that is closely matched to the CTE of the substrate being over molded. In most cases, however, the substrate is formed from a metal, which typically has a CTE in the range of between 11 ppm/° C. and 27 ppm/° C. Accordingly, the polymer composition that is utilized for over molding need only have a CTE that also falls within this range. For example, U.S. Pat. No. 4,831,480 discloses a carriage apparatus wherein a magnetic head mounted onto a die-cast metal substrate is over molded with a thermally conductive polymer wherein the die-cast metal has a CTE of approximately 24 ppm/° C. In this case, it was relatively easy to produce a polymer composition having a CTE in the desired range because most of the common base polymers have a native CTE in the desired range. Liquid crystal polymer has native CTE properties of between 3 ppm/° C. and 17 ppm/° C. when manufactured in thin films and approximately 30 ppm/° C. when utilized to product three-dimensional objects. PTFE has a CTE range of between 9 ppm/° C. and 12 ppm/° C. Polyamide has a CTE of approximately 20 ppm/° C. Accordingly, virtually any filler in the loading ranges disclosed would produce a finished composition having a CTE that is closely matched to the substrate CTE value of between 11 ppm/° C. and 24 ppm/° C.
In an alternative approach, U.S. Pat. No. 6,600,633 discloses an actuator for an optical disk drive, wherein a coil assembly is over molded utilizing a thermally conductive liquid crystal polymer resin. In this case the concern is not principally directed towards the matching of the CTE's between the substrate and the over mold materials but instead attempts to keep the operating temperature of the overall assembly low enough to prevent it from reaching a point where the mismatched CTE's result in differential stresses. In this reference, the composition is simply formulated to have a thermal conductivity value that is high enough to maintain the substrate at an operational temperature that is below the critical range wherein differential CTE properties become problematic.
While both of these disclosures are well suited for their particular application, none of the prior art references provide a solution that is suitable for applications wherein a high precision task requires extreme dimensional stability at greatly elevated operational temperature ranges. For example, FIG. 1 illustrates a print head for high precision printers that utilize hundreds of ink nozzles and/or miniature heaters 1, all of which are positioned in closely spaced arrays on single substrate material 2. During operation each of the heaters 1 is utilized to either disperse the ink from the nozzle (ink dispersion type printers) or cause ink located on an adjacent ink carrier ribbon to be transferred (thermal transfer type printers). As can be appreciated, during any given print operation a large amount of waste heat is produced within the print head array that must be dissipated. Should the waste heat not be effectively dissipated, it will accumulate in the substrate 2 of the print head. In cases wherein the substrate 2 used to support the print head has a relatively large CTE, the substrate 2 will ultimately expand to a point wherein the print quality will be dramatically decreased. Accordingly, most print head manufacturers have utilized ceramic substrates 2 with CTE values in the range of between 2 ppm/° C. and 9 ppm/° C., nearly a full order of magnitude lower than the CTE values discussed above with respect to metallic substrates. Further, while an example is provided that discloses a print head, the underlying issue impacts any electronic assembly wherein a high degree of dimensional stability is required even at elevated operational temperature ranges.
To assist in dissipating the waste heat encountered in such applications, heat sinks 3 have been developed to provide additional surface area and heat dissipating volume through which the waste heat can be managed. Generally, such heat sinks 3 are affixed to the substrate 2 using an adhesive layer 4. The problem is that machined metallic heat sinks 3 having CTE values that are much greater than the ceramic substrates 2 tend to expand at a higher rate than the substrate to which they are mounted resulting in the introduction of differential stresses in the substrate 2 and frequent failure of the bond 4 between the heat sink 3 and the substrate 2. Further, metallic heat sinks 3 are relatively expensive to manufacture requiring extensive machining to produce the desired heat sink geometry. Similarly, the thermally conductive polymers utilized to over mold heat sinks 3 in the prior art exhibit undesirable CTE properties that are also a full order of magnitude greater than the CTE values of the ceramic substrates 2 utilized in such high precision electronics devices. In addition, as discussed above, where the operation temperature range of the assembly is much lower, a polymer solution works because it succeeds in transferring sufficient heat away from the assembly substrate 2 before the overall device reaches a temperature wherein differential CTE properties become problematic. In the types of applications anticipated within the present application, it is simply not possible to maintain an operating temperature at such a low threshold. As a result, the industry to date has resorted to forming the substrates 2 and the associated heat sinks 3 from the same ceramic materials in order to achieve the necessary dimensional stability and thermal conductivity necessary to maintain the precision that is critical in such applications.
While forming the substrates and heat sinks from ceramics has produced a high precision and highly thermally conductive assembly, the costs associated with manufacturing such an assembly is dramatically higher than other conventional manufacturing methods. When manufacturing ceramics, the materials must be compression molded and sintered to solidify the base materials before the assembly can be removed from the mold. This type of molding process is time consuming and costly, in addition to the fact that the overall molding process results in a large amount of part shrinkage, wherein further anti-compaction and shrink reduction agents must be added to the base composition to overcome some of the undesirable shrinkage.
In view of the foregoing, there is a need for a thermally conductive polymer composition, which has properties that are complimentary to the properties of the low CTE materials utilized in high precision operations. There is a further need for a thermally conductive polymer composition that has CTE values in the ranges typically encountered in ceramic materials thereby providing the dimensional stability required for such high precision operations. Finally, there is a need for a thermally conductive polymer having ultra low CTE properties while also being net-shape moldable, thereby reducing the overall cost to manufacture such high precision components.